1. Field of the Invention
The present invention relates to a magnetic rubber encoder for use in detecting the number of revolution of a rotating element.
2. Description of the Prior Art
Certain magnetic rubber encoders for detecting the number of revolution of a rotating element or the like includes a rubber magnet assembly which has a rubber magnet rigidly secured to an annular core metal and having a plurality of opposite magnetic poles magnetized thereto so as to alternate with each other in a direction circumferentially of the core metal. When in use, the magnetic rubber encoder cooperates with a magnetic sensor, disposed in face-to-face relation therewith, so that as the magnetic rubber encoder rotates together with a rotating element, the magnetic sensor can detect passage of the opposite magnetic poles and then provide an electric output indicative of the number of revolutions of the rotating element in terms of the frequency of the opposite magnetic poles having traversed in front of the magnetic sensor.
In order to enhance the resolution of the magnetic rubber encoder, the polarization pitch on a magnetic member made of a rubber magnet, that is, the polarization pitch of each of the alternating magnetic poles must be reduced as small as possible. FIG. 2 illustrates the relationship between the polarization pitch p and the magnetization intensity b, and it is clear that if the polarization pitch p decreases in order to enhance the resolution, the magnetization intensity b decreases correspondingly.
The Japanese Laid-open Patent Publication No. 3-130815 discloses determination of the optimum thickness of a magnetized rubber to obtain the maximum magnetization intensity bmax, by optimizing the ratio between the thickness w of the magnetized rubber and the polarization pitch p when the latter is within the range of about 0.25 mm to 2.5 mm. According to this patent publication, the optimum thickness w of the magnetized rubber is calculated about 1.25 p. with the ratio between the thickness wand the polarization pitch p chosen to be within the range of 1.25±25%.
However, when extensive studies were conducted, in which the polarization pitch p and the magnetized rubber thickness w were changed, it has been found that no maximum magnetization intensity bmax could be obtained when the thickness w of the magnetized rubber encoder was about 1.25 times the polarization pitch p, with the polarization pitch p within the range of about 0.5 mm to 1.5 mm.